Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V21: Spin-Photon Coupling in Semiconductor Quantum DotsFocus
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Sponsoring Units: GMAG DMP FIAP DCOMP Chair: Guido Burkard, Univ Konstanz Room: LACC 309 |
Thursday, March 8, 2018 2:30PM - 3:06PM |
V21.00001: Strong-coupling Cavity QED with Single Electron Charge and Spin Qubits in Silicon1 Invited Speaker: Xiao Mi Coherent coupling of single qubits to microwave photons provides a scalable pathway toward long-range qubit-qubit entanglement. We first demonstrate strong-coupling between a single electron charge qubit in a gate-defined Si double quantum dot to a microwave photon in a superconducting cavity2. Combining electric-dipole interaction with spin-charge hybridization in the presence of a magnetic field gradient, we also achieve the strong-coupling regime between a single electron spin and a single microwave photon3. Spin-photon coupling rates up to 11 MHz are supported by the device architecture, exceeding direct magnetic-dipole coupling rates by five orders of magnitude. Furthermore, we demonstrate all-electric control and quantum non-demolition readout of the single-spin qubit using its dispersive interaction with the microwave cavity. These results allow for the construction of a large-scale Si quantum processor with “all-to-all” connectivity. |
Thursday, March 8, 2018 3:06PM - 3:18PM |
V21.00002: Tunable Few-electron Charge and Spin States in Parallel-Coupled Quantum Dots in InAs Nanowires Malin Nilsson, Florinda Viñas Boström, Sebastian Lehmann, I-Ju Chen, Martin Leijnse, Kimberly Dick Thelander, Claes Thelander High-resolution spin spectroscopy is performed on parallel-coupled double quantum dots in the few-electron regime. The system consist of a InAs nanowire of zinc blende crystal structure, having a single quantum dot (QD) epitaxially defined by two thin segments of wurtzite, acting as tunnel barriers (offset ~100 meV) [M. Nilsson et al, PRB 93, (2016)]. The small axial extension of the QD (<10 nm) leads to a strong quantum confinement and enables the QD to be fully depleted of electrons. By using pairs of local side gates and a global back gate the system can be tuned from one QD into parallel double QDs, for which we can control the populations down to the last electrons. The combination of hard-wall barriers to source and drain, shallow inter-dot tunnel barriers, and very high single-QD excitation energies (up to 27 meV), allow an order of magnitude tuning of the strength for the first intramolecular bond. In addition, the consistently large |g*|-factors (~9) facilitate detailed studies of the B-field dependence of the 1- and 2-electron states. Specifically, we find that it is possible to tune the magnitude of the B-field induced singlet-triplet anti-crossing by changing the inter-dot tunnel coupling. We model the experimental data using a simple few-electron Hamiltonian. |
Thursday, March 8, 2018 3:18PM - 3:30PM |
V21.00003: Input-Output Theory for Spin-Photon Coupling in Si Double Quantum Dots Monica Benito, Xiao Mi, Jacob Taylor, Jason Petta, Guido Burkard The interaction of qubits via microwave frequency photons enables long-distance qubit-qubit coupling and facilitates the realization of a large-scale quantum processor. However, qubits based on electron spins in semiconductor quantum dots have proven challenging to couple to microwave photons. In this theoretical work [1] we show that a sizable coupling for a single electron spin is possible via spin-charge hybridization using a magnetic field gradient in a silicon double quantum dot. Based on parameters already shown in recent experiments, we predict optimal working points to achieve a coherent spin-photon coupling. Our predictions are in good agreement with recent measurements [2] which demonstrate strong coupling with spin-photon coupling rates of more than 10 MHz. Furthermore, we employ input-output theory to identify observable signatures in the cavity output field, which can provide guidance to the experimental search for strong coupling in such systems and opens the way to cavity-based readout of the spin qubit. |
Thursday, March 8, 2018 3:30PM - 3:42PM |
V21.00004: Spin orbit interaction in dynamics of silicon double quantum dots Ernesto Cota, Sergio Ulloa We present a theoretical study of the role of spin-orbit interactions in a silicon double quantum dot. We propose that an accurate estimate of the strength of this interaction can be obtained through the study of the return probability of the double occupation singlet state in a magnetic field, as the system is gated dynamically across the relevant states in the low energy two-electron manifold. Landau-Zener-Stückelberg (LZS) type of processes involving appropriate control of voltage pulses across neighboring avoided crossings in the energy spectrum of the system are utilized to explore the system dynamics. Our description takes into account Zeeman splitting, intervalley mixing and spin-orbit interaction present in the structure. Using a density matrix equation of motion approach, we carry out numerical calculations of the return probability of the double occupation singlet state. The analysis in terms of LZS theory allows the determination of the spin-orbit coupling strength for different Zeeman splitting regimes. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V21.00005: Optical Dependence of EDMR in Silicon Devices Lihuang Zhu, Kipp van Schooten, Mallory Guy, Chandrasekhar Ramanathan Electrically-detected magnetic resonance (EDMR) provides a highly sensitive method for reading out the state of donor spins in silicon. The technique relies on a spin-dependent recombination (SDR) process involving dopant spins that are coupled to interfacial defect spins near the Si/SiO2 interface. At cryogenic temperatures and low-doping concentrations, optical excitation is used to generate the free carriers. We investigate the wavelength dependence of the EDMR signal in a Si:P device. With near-infrared excitation we find that the EDMR signal primarily arises from donor-defect pairs, while at higher photon energies there are significant additional contributions from defect-defect pairs. At longer wavelengths, the contribution of defect spins adjacent to the buried oxide layer also increased due to the increased penetration depth into the device. Careful tuning of the optical excitation energy allows us to control both the SDR dynamics and to characterize depth-dependent features of the EDMR signal. |
Thursday, March 8, 2018 3:54PM - 4:06PM |
V21.00006: Effect of Modified Periodic Waveforms on Current-Induced Spin Polarization Measurements Joseph Iafrate, Davide Del Gaudio, Simon Huang, Rachel Goldman, Vanessa Sih Applying a time-varying periodic voltage to a semiconductor sample generates a current-induced electron spin polarization (CISP). Using an ultrafast mode-locked laser and lock-in detection scheme, we measure CISP on a 500nm indium gallium arsenide epilayer (2.6% indium concentration) grown on a (001) gallium arsenide substrate via Faraday rotation and extract the spin generation rate. While the measured spin polarization initially increases linearly with electric field as observed in previous work, larger applied voltages lead to sample heating and a decreasing spin generation rate. We modify the applied voltage waveform to reduce heating, requiring that we add an extra data processing step to our measurement technique. We then recover the linear dependence of spin generation rate with electric field even at larger applied voltages. Future CISP studies can utilize this technique to investigate CISP under larger applied electric fields. |
Thursday, March 8, 2018 4:06PM - 4:18PM |
V21.00007: Quantum Decoherence in Reduced Dimensions Meng Ye, Giulia Galli The electronic spin states of certain point defects in solid state materials have been shown to be promising quantum bits (qubit) for quantum information processes. Long spin coherence time is an important criterion to meet, in order to realize a qubit. In the presence of an external magnetic field and at low temperatures, decoherence originates mainly from the temporally fluctuating random magnetic field induced by nuclear spin flip-flops. This process can be theoretically described by a spin Hamiltonian and approximately solved by a cluster-correlation expansion (CCE) method [1]. Recently, single-photo emitters have been discovered in several reduced dimensional systems, such as 2D hexagonal-BN. In this work, we used the CCE method to study the decoherence of a electron spin immersed in a nuclear spin bath as a function of dimensionality. We considered 3D bulk materials, thin films, and 2D materials. Our results shed light on the importance of dimensionality in determining the spin decoherence dynamics of spin defects and provide predictions about promising solid-state qubits in low dimensional systems. |
Thursday, March 8, 2018 4:18PM - 4:30PM |
V21.00008: Spin effects in quench dynamics of quantum dots and molecules Ireneusz Weymann, Kacper Wrzesniewski We study the quench dynamics of correlated quantum dots and molecules attached to spin-polarized leads. We focus on the strong coupling regime, where electron correlations can give rise to the Kondo effect. We study the situations in which the quench is performed either in the coupling to external electrodes or in the position of the orbital level. The time-evolution of expectation values of local occupation and magnetization indicates a destructive role of the proximity induced exchange field on the Kondo state. For single-level quantum dots this takes place out of the particle-hole symmetry point, while for molecules with larger spin detrimental effect of exchange field occurs in the whole Kondo regime. We also analyze the relevant time scales for the development of corresponding exchange fields and study their dependence on temperature, coupling strength and position of orbital level. The analysis is done with the aid of the time-dependent density-matrix numerical renormalization group method. |
Thursday, March 8, 2018 4:30PM - 4:42PM |
V21.00009: Designing defect-based qubit candidates in wide-gap binary semiconductors Hosung Seo, He Ma, Marco Govoni, Giulia Galli The development of novel quantum bits is key to extend the scope of solid-state quantum information science and technology. Here, using first-principles calculations, we propose that large metal ion - vacancy complexes are promising qubit candidates in two binary crystals: 4H-SiC and w-AlN. In particular, we found that the formation of neutral Hf- and Zr-vacancy complexes is energetically favorable in both solids; these defects have spin-triplet ground states and electronic structures similar to those of the diamond NV center and the SiC di-vacancy. Interestingly, they exhibit different spin-strain coupling characteristics, and the nature of heavy metal ions may ensure stability against defect diffusion. In order to support future experimental identification of the proposed defects, we report predictions of their optical zero-phonon line, zero-field splitting and hyperfine parameters. The defect design concept identified here may be generalized to other binary semiconductors to facilitate the exploration of new solid-state qubits. |
Thursday, March 8, 2018 4:42PM - 4:54PM |
V21.00010: Toward ultrafast spin lasers? Gaofeng Xu, Nils Gerhardt, Igor Zutic Injecting spin-polarized carriers in semiconductor lasers provides operating principles for room temperature spintronic devices not limited to magnetoresistance. Important steady-state properties have been demonstrated in these lasers, including threshold reduction [1] and spin amplification [2]. However, their main advantage is dynamical operation, predicted to have an enhanced modulation bandwidth, improved switching, and faster operation than the conventional lasers (with spin-unpolarized carriers) [3-5]. As these predictions are being verified [6,7], we provide a generalized description of spin lasers to interpret related experiments and understand the limiting factors for their operation. [1] J. Rudolph, et al., Appl. Phys. Lett. 87, 241117 (2005). [2] S. Iba, et al., Appl. Phys. Lett. 98, 081113 (2011). [3] P. E. Faria Junior, G. Xu, J. Lee, N. C. Gerhardt, G. M. Sipahi, and I. Zutic, Phys. Rev. B 92, 075311 (2015). [4] J. Lee, R. Oszwaldowski, C. Gothgen, and I. Zutic, Phys. Rev.B 85, 045314 (2012). [5] E. Wasner,S. Bearden, J. Lee and I. Zutic, Appl. Phys. Lett. 107, 082406 (2015). [6] M. Lindemann, T. Pusch, R. Michalzik, N. C. Gerhardt,and M. R. Hofmann, Appl. Phys. Lett. 108, 042404 (2016). [7] H. Hopfner, et al., Appl. Phys. Lett. 104, 022409 (2014). |
Thursday, March 8, 2018 4:54PM - 5:06PM |
V21.00011: Probing the Coherent Spin Dynamics of a Magnetic Impurity in a III-V Semiconductor Stephen McMillan, Nicholas Harmon, Michael Flatté Individual magnetic impurities or small collections of magnetic impurities in III-V semiconductors can be identified via scanning tunneling microscopy (STM)[1,2], their exchange interaction can be measured [3], and they can have remarkably long spin coherence times[4]. Coherent spin dynamics, however, has only been probed in ensemble measurements [4,5]. We describe an approach to explore this coherent spin dynamics through low-field magnetoresistance involving a spin-polarized STM contact. Measurements of the spin coherence time and the local hyperfine interaction should be feasible. We also describe the effect of exchange interactions between magnetic impurities on the magnetoresistance. |
Thursday, March 8, 2018 5:06PM - 5:18PM |
V21.00012: Optical spectral weight for the magneto-optical conductivity of topological spintronic semiconductors Zhou Li The Fermi velocity (vF) associated with the spin-orbit coupling is two orders of magnitude smaller for spintronic semiconductors than it is for topological insulators. Both families can be treated with the same Hamiltonian which contains a relativistic (Dirac) linear in momentum term proportional to vF and a nonrelativistic quadratic contribution with Schrödinger mass (m). We find that the ac dynamic longitudinal and transverse (Hall) magnetoconductivities are strongly dependent on the size of vF. When the Dirac fermi velocity is small, the absorption background provided by the interband optical transitions is finite only over a very limited range of photon energies as compared with topological insulators. Its onset depends on the value of the chemical potential (μ) and on the magnetic field (B), as does its upper cutoff. Within this limited range its magnitude is, however, constant and has the same magnitude of e2π/(8h) as is found in topological insulators and also in graphene noting a difference in degeneracy factor. |
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